The potent and widespread vascular actions of purine nucleotides and nucleosides have long been recognized. Naturally occurring extracellular purine nucleotides and nucleosides exert cardiovascular responses by stimulating various P1 and P2 receptors [1,2]. Adenine nucleotides and nucleosides are used for many diagnostic and treatment purposes in daily clinical practice such as assessment of coronary blood flow [3-8] and as anti-arrhythmic agents. Adenosine non-selectively activates 4 receptor subtypes: A1, A2A, A2B, and A3. Activation of cardiac A2A and A2B adenosine receptors vasodilates the coronary and peripheral arterial beds, increases myocardial blood flow (MBF), and causes sympathoexcitation, but also results in mast cell degranulation and bronchial constriction. Nevertheless, intracoronary and intravenous adenosine are employed in the clinic for assessment of fractional flow reserve (FFR).
Uridine 5-triphosphate (UTP) is also a naturally occurring compound in the circulation and is discharged during acute myocardial infarction. UTP stimulates P2Y2 and P2Y4 receptors, where the first are predominant in the human cardiovascular tree. UTP is highly selective for this receptor.
Previous assumptions underlying the use of UTP in treatment of cardiovascular disease have proved inaccurate in vivo. It was demonstrated in 2004 and 2008 [9,10], by using local infusion of ATP and UTP, that these two agents were the only registered metabolites capable of opposing sympathetic vasoconstriction and concomitantly increasing blood flow (85% and 60% of maximum in the leg during hard exercise, respectively). However, the present inventor, using systemic infusion of UTP in pigs, has found that UTP can only maximally lower mean arterial pressure by 30% (Example 4), which is substantially less than that achieved by ATP because ATP can produce limitless lowering of blood pressure. Therefore, ATP is more potent than UTP systemically as well as locally in the healthy leg, although both ATP and UTP were shown previously to be equipotent in the peripheral vascular system for the P2Y2 receptor in the arms of humans [11].
The higher potency of ATP compared to UTP in the normal healthy leg was attributed to the degradation of ATP to ADP, AMP, and adenosine. These degradation products, in conjunction with ATP, could contribute to elevate blood flow higher than that achieved by UTP, which does not have any vasoactive degradation products. Thus, a comparison of the relative vasoactive potencies of exogenous nucleotides and adenosine revealed the following rank order: ATP (100)=UTP (100)>>adenosine (5.8)>ADP (2.7)>AMP (1.7), but only for blood flows around 3.5 Lmin−1 [9].
Comparative studies in the human leg of healthy and diabetic patients showed that UTP>>ATP with regard to vasodilation in legs of elderly patients and patients with type 2 diabetes [12]. This is clearly in contrast to previous findings, but is more clinically relevant as most cardiac patients are older. Interestingly, the discrepancy in vasoactive potency was not due to an up-regulation of the P2Y2 receptor, suggesting that up-regulation of other ATP-related (but not UTP) vasoconstrictive receptors must be relevant when people are of advanced age and poor general health. However, recent studies have shown that the ability to oppose sympathetic vasoconstriction during exercise is intact in patients with type 2 diabetes, suggesting that previous assumptions of a crucial involvement of the P2Y2 receptor in functional sympatholysis are inaccurate [9]. This conclusion is also supported by another study by the present inventor in pigs where ADP or UTP was infused during myocardial infarction. In that study, UTP increased the infarct area during acute myocardial infarction, whereas ADP diminished it (unpublished results). This suggests a pharmacological cardioprotective effect of ADP, but a detrimental effect of UTP, suggesting that caution must be exercised when using UTP in clinically acute conditions. Therefore, the previous assumptions in patent application WO 2007/065437 relating to the regulation of purinergic receptor activity to modulate vascular tone, particularly for treating hemodynamic conditions by overriding vasoconstriction activity, which could potentially have clinical applications for treatment with UTP agonists or antagonists, is incorrect.
In contrast to the vasodilating properties associated with UTP mentioned above, UTP has also been described to be a potent vasoconstrictor in the coronary circulation. Of particular note are the studies relating to human coronary arteries and bypass vessels [13-15]. In these studies, both UTP and UTPγS induced contractions in coronary arteries and the internal mammary artery in heart transplant patients, suggesting that P2Y2 receptors are important contractile receptors. UTPγS also induced contractions in the saphenous vein. Given that prolonged treatment with UTP has been shown to induce smooth muscle proliferation in vitro, it can be speculated that in case of endothelial dysfunction (such as with coronary artery disease), extracellular nucleotides derived from the blood may reach smooth muscle cells (SMCs), leading to UTP-mediated vasoconstriction of P2Y2 receptors. In human coronary arteries, the P2Y2-subtype has been presumed to play a major role in this speculated constriction [14,15]. This is also seen in animal studies [16,17]. In pigs, the P2Y2 receptor is up-regulated in SMCs of in vivo stented coronary arteries to mediate the mitogenic effects of nucleotides [18]. Therefore, the P2Y2 receptors are suspected to take part in the pathophysiological genesis of potentially life-threatening vasospasms [15].
Besides being looked at as evoking coronary vasoconstriction of damaged vessels, extracellular nucleotides have also been implicated to play an important role in the development of inflammatory vascular disease [19,20]. Seye et al. showed that UTP, acting at P2Y2 receptors, promoted intimal hyperplasia of collared rabbit carotid arteries [21]. In an animal model, porcine P2Y2 receptors were found to be overexpressed in stented coronaries and to play a distinct mitogenic role there [18]. It has thus been accepted that vascular remodeling, facilitated by extracellular nucleotides, is a key step in the genesis of cardiovascular and cerebrovascular disease, potentially culminating in life-threatening states of stroke or heart attack. Therefore, no preceding clinical studies have ever been attempted involving in vivo coronary infusion of UTP in humans, because this compound, for all the above mentioned reasons, was believed to be hazardous to humans.
Angiographic assessment of coronary artery disease (CAD) has guided cardiac therapy for more than 30 years, however even experienced angiographers are unable to reliably assess lesion severity because angiography has significant intra-observer and inter-observer variability and is not a physiological assessment, but merely a visual one. Recent studies, such as the COURAGE trial, have re-emphasized what all current medical guidelines recommend: that for low risk patients, even those experiencing angina, optimal medical therapy should be the initial treatment. For those patients whose disease progresses, or for whom chest pain is not alleviated, revascularization, either through angioplasty and stenting or surgery, should be performed.
The new diagnostic tool fractional Flow Reserve (FFR) helps physicians to decide whether to intervene on a stenosis (i.e., abnormal narrowing of blood vessels) or not. Achievement of maximal hyperemia of coronary microcirculation is a prerequisite for the exact assessment of FFR in order to minimize the effect of microvascular resistance. Thus, the higher the flow, the larger the pressure drop across the stenotic vessel, i.e., the lower the FFR. For accurate FFR measurements, achievement of maximal hyperemia is imperative for minimizing the effect of microvascular resistance. Only at maximal hyperemia is flow and pressure linearly correlated. If there is only suboptimal hyperemia, the FFR index underestimates the functional severity of coronary stenosis. This can lead to injurious outcomes [22]. Therefore, it is crucial for clinical decision making, that the FFR response is accurate, otherwise over- or under-treatment of patients will occur, leading to higher mortality rates and more expensive treatment regimens.
The preferred standard hyperemic agent used today for inducing coronary hyperemia is adenosine. However, adenosine use is associated with side effects even with local infusions. For example, adenosine causes dyspnea and angina in nearly all patients, as well as second-degree AV block in some patients. Adenosine use is also associated with contraindications such as asthma, COPD, angina, hypotension, 2nd or 3rd-degree AV block, and sinus node dysfunction; and the need for abstinence from caffeine in order to get an accurate hyperemic assessment, because caffeine blocks P1 receptors, which are the vasodilatatory receptors the adenine compounds function through [23].
Given the foregoing limitations associated with the use of adenine-related compounds (e.g., adenosine and ATP) as hyperemic agents, more potent hyperemic agents with fewer side effects would be beneficial for diagnosing compromised blood flow in blood vessels.